Part 1. The Field of the Invention
This invention relates to novel, improved ink compositions and to writing instruments including such ink compositions. More precisely, the invention disclosed herein relates to aqueous ink compositions including water dispersible, polymeric shear-thinning providing materials and to writing instruments including the ink compositions.
Part 2. Description of the Prior Art
Ink compositions including water dispersible, polymeric, shear-thinning providing materials are disclosed in commonly assigned, copending application Ser. No. 693,930 filed Jan. 23, 1985 by Laura K. Case et al. application Ser. No. 693,930 is expressly incorporated herein in its entirety by reference. Essentially, the ink compositions disclosed in application Ser. No. 693,930 are thickened liquids at rest. They may have a rheological yield value and are highly pseudoplastic or shear-thinning in use. In other words, they become thin, readily flowable liquids having a viscosity of no greater than about 100 m Pa.s at shear rates produced in writing such as with a ball point pen. The ink compositions include at least one water dispersible, polymeric shear-thinning providing material uniformly dispersed in a highly polar solvent system which is preferably water or primarily water. Other materials such as coloring materials, surfactants, preservatives, corrosion inhibitors, humectants and the like can also he included in the inks. For the purposes of this invention, a water dispersible, polymeric shear-thinning providing material or shear-thinning material means those polymeric materials described in application Ser. No. 693,930 which can be dispersed in a highly polar solvent system to provide an ink having a shear-thinning index "n" of between about 0.01 to about 0.60. The shear-thinning index (n) is calculated by fitting shear stress (t) and shear rate (j) values (obtained from rheological measurements on a viscometer such as a Haake Rotovisco, Haake Inc. Saddle Brook, N.J.) to the empirical power law equation t=Kj.sup.n (K and n are calculated constants).
The preferred shear-thinning inks of U.S. Pat. application Ser. No. 693,930 include polymeric shear-thinning providing materials which are substantially water soluble. Accordingly on drying, the compositions provide a coalesced residue which is substantially water soluble and which exhibits a low degree of water-fastness. Moreover the shear-thinning inks have relatively high viscosities at low shear rates and, like other high viscosity inks, are susceptible or vulnerable to the effects of shock. Normally, the shear-thinning inks disclosed in U.S. application Ser. No. 693,930 are contained in hollow, non-pressurized tubes and fed directly to the nib or ball of the writing instrument. However, when the ink is contained in a non-pressurized tube, the instrument should be handled carefully to avoid shock. For example, if the instrument is dropped, tapped against a hard object or otherwise jarred, the column of ink in the tube may separate or be pulled or pushed away from the region of contact between the column of ink and the nib or ball. This separation can result in a pocket of air being established between the nib and the column of ink which can affect the writing performance of the instrument until contact between the ink column and the nib is restored.
As those in the art know, a high viscosity ink can be retained in hollow, pressurized tubes in order to control the effects of shock. However, pressurized tubes can present unattractive manufacturing cost features in terms of materials and assembly particularly for low priced, disposable writing instruments. This invention is addressed to the problems of controlling the effects of shock on high viscosity ink compositions and provides an especially effective solution to those problems.